Welcome to the quantum internet: quantum encryption is here, but the laws of physics can do much more than protect privacy

Science News, August 16, 2008 by Davide Castelvecchi

A stylish new way of surfing the Internet is coming to Vienna this fall. Researchers plan to flip the switch on the next step toward a quantum version of the Internet. They will build a network allowing users to send each other messages as virtually unbreakable ciphers, with privacy protected by the laws of quantum physics.

The Vienna net is admittedly just a prototype for research purposes. It is also not yet a true quantum version of the Internet. Although it can transmit ordinary data with quantum security, it can't transfer quantum information, which encodes the states of objects that obey quantum rules. Such a breakthrough might be years off, but it's getting closer.

Truth be told, it's not completely clear what a fully quantum Internet would be good for. In fact, at first it even sounds like a really bad idea. Quantum information is notoriously wobbly. An object tends to live in a superposition of states--for example, an electron can spin in two directions at once, or an atom can be simultaneously in two different places--until interaction with the rest of the world forces the object to pick one state. Quantum reality is a limbo of coexisting possibilities.

And because any measurement done of a quantum system changes the system's state irreversibly, quantum information is different every time it's read. That makes it impossible, for example, to copy, broadcast or back up quantum data.

But the eccentric physics could also impart unique strengths to networks. While each data bit in an ordinary computer takes the value 0 or 1, the units of quantum information, called quantum bits, or qubits for short, can take both values simultaneously. A quantum Internet could transfer software and data between future (and futuristic) quantum computers, which could outperform ordinary computers by running multiple operations at once, in superposition. And the network could lead to new kinds of social interactions--such as letting quantum physics pick a presidential candidate who pleases the most voters or allowing people to donate to a cause based on whether others donate as well--and do so with absolute secrecy.

Perhaps--and this inches toward Star Trek territory--some day a quantum net could even "beam up" a physical object. All the information needed to re-create the object, such as its shape and energy, would be transferred elsewhere, leaving just chaos behind.

In the meantime, when the switch is flipped October 8, the Vienna net will demonstrate how quantum physics can keep ordinary information, such as an e-mail or the balance of a checking account, safe from prying eyes.

This latest step toward the quantum Internet is a limited network backbone that will often run at the speed of a 1980s modern. To plug into it, a user would need to buy expensive gear and link an optical fiber to one of the backbone's five nodes. But it's a step.

Meanwhile, most of the basic technical ingredients of a truly quantum Internet have now been demonstrated, at least in the lab. In particular, researchers have created various types of "quantum memory," in which light pulses traveling through an optical fiber essentially slow to a halt, a crucial requirement for the quantum version of an Internet router. So it may be just a matter of time before scientists can start beaming up stuff--or at least data.

"I'm optimistic that within a few years we'll be able to build at least a lab demonstration of a quantum network," says Mikhail Lukin of Harvard University.

A solid quantum key

In tunnels stretching under Vienna and the Danube river, pulses of light will be beamed this October along tens of kilometers of existing optical fibers owned by German engineering conglomerate Siemens. A collaboration of more than 40 universities, companies and research institutions piece together technologies to link five Siemens buildings, four of them scattered across the city and one 85 kilometers away in the town of St. Polten.

The building-to-building connections will use a number of quantum encryption systems to pass the information, many of them inspired by a version of quantum encryption first proposed in 1991 by Artur Ekert, now at the National University of Singapore. With Ekert's procedure, the sender and the receiver, conventionally called Mice and Bob, use both a quantum connection and a classical one, which could be the good-old Internet or a phone line.

Through the quantum connection, Mice and Bob establish a common encryption key--a secret sequence of data bits that Mice will use to scramble her message, and Bob to unscramble it. Mice can then send her scrambled message to Bob through the classical connection, for example as an e-mail attachment.

To someone who doesn't know the key, Alice's message would look like a random sequence of bits. Even the most sophisticated computer imaginable wouldn't be able to crack it. But Bob knows the key, so he can unscramble the message.

Keeping the key secret as they create it is the crucial part, and here's where Ekert exploits quantum physics--specifically, a weird phenomenon called quantum entanglement. In quantum physics, each of two objects can exist in its own state, or the objects' states can be entangled, meaning that, while separate, they are not independent of each other.